GTA Online's Diamond Casino (opens in new tab) has opened its doors to the masses and their bank accounts. You can find the flashy building in Vinewood, where you'll be able to play poker, blackjack, and roulette. You can even bet on the horses.
To start gambling, you'll need to exchange cash for chips at cashier services, though you'll also get 5000 right away for getting membership at the casino (this costs $500 of in-game currency). You'll also get free chips each day. The exchange rate is 1 chip for $1 of in-game currency, so I'm not sure why you need chips at all, but those are the rules (edit: you can seemingly only buy 50,000 chips at a time before a cooldown timer occurs).
If you're lucky enough to win big, you can just give it all back to the casino by splashing out in the casino store, where you can buy art and golden guns. Selling firearms in a casino? Very GTA.
If your cash is really burning a hole in your pocket, you can spend it all on The Diamond penthouse. There's an office, a media room, a spa, multiple bedrooms, a private bar—it's very fancy, and you can customise it.
Apparently it will cost you $63,548,081 (cheers, UnknownModder) to buy absolutely everything in the DLC. That's a lot of hours on the slot machines. They're pretty cool slot machines, though. Some of them have moustaches!
Twitch Prime subscribers who connected their Rockstar Social Club accounts before the 19th get $1.5 million off any penthouse, so you can pick up the most basic one for free. You'll need to log in, log out and then log in again to get your Prime bonus. Everyone else will need to make a lot of cash in the casino or do some more crimes.
The casino has actually been around since GTA 5 launched, and it's been 'opening soon' for around six years. Go try your luck now.